Originally, a crankshaft drove the front end assembly drive (FEAD) system of an engine. The crankshaft was turned by the firing of pistons, which exerted a rhythmic torque on the crankshaft, rather than being continuous. This constant application and release of torque caused vacillations, which would stress the crankshaft to the point of failure. Stated another way the crankshaft is like a plain torsion-bar, which has a mass and a torsional spring rate, that causes the crankshaft to have its own torsional resonant frequency. The torque peaks and valleys plus the inertia load from the acceleration of the reciprocating components causes the crankshaft itself to deflect (rotationally) forward and backward while it is operating. When those pulses are near the crankshaft resonant frequency, the crankshaft vibrates uncontrollably and will eventually break. Accordingly, a torsional vibration damper (sometimes referred to as a crankshaft damper) is mounted on the crankshaft to solve this problem by counteracting torque to the crankshaft, thereby negating the torque twisting amplitude placed upon the crankshaft by periodic firing impulses, and to transfer rotational motion into the FEAD system, typically by driving an endless power transmission belt.
While existing torsional vibration dampers have been effective to extend the life of the crankshaft and to drive the FEAD system, changes in vehicle engine operation, such as the introduction of start-stop systems to conserve fuel consumption, add complexities to the system that the existing torsional vibration dampers are not designed to address. For instance, the start-stop system introduces impact forces due to belt starts that introduce potential slip in the elastomer-metal interface in traditional torsion vibration dampers. Another concern is maintaining good axial and radial run-outs between the metallic components.
Some torsional vibration dampers also include an isolator system. Some of these isolator systems use an elastomeric rubber spring, which provides a highly non-linear spring rate. However, these elastomer-based isolator systems tend to fail after prolonged use, and accordingly have a finite fatigue life. Isolators with elastomer springs also have high temperature vs. frequency dependence, which means that the performance of elastomer springs may vary depending upon the temperature. The elastomer spring material tends to be “softer” at lower temperatures than at higher temperatures, which changes the elastomeric attributes of the material. Accordingly, elastomeric springs are typically designed to operate within a nominal temperature range, and, in some cases, may not function properly when used under temperature conditions that vary from the nominal temperature range.
Other isolator systems use mechanical springs, which provide a large free angle for vehicle start/stop. However, these systems tend to produce undesirable audible noise as the metallic spring rubs against its carriage. To alleviate the noise, it is common to pack the spring cavity with grease. These mechanical spring-based isolators tend to be heavy because of the weight of the springs, the seating arrangement, etc. These systems can also be expensive in terms of both material and manufacturing costs (for example, the spring is often nitrided for wear resistance against the spring cavity).
Accordingly, improved designs for torsional vibration dampers having isolators, which may also be referred to as isolators, decouplers, or pulleys, are needed which are relatively quiet in operation, light and compact in construction, and inexpensive, yet which provide a relatively large free angle for vehicle start/stop and non-linear spring functionality.